Interesting point. Of course it's not something my Japanese teachers ever said - understandable I guess ' they wanted us to say it correctly. In those circumstances including such readings with an "ik" tag is probably appropriate.

jpwiki: 20日（はつか、にじゅうにち）は、暦上の各月における20日目である。

I think the big difference between the reading にじゅうにち and entries of the ろにん, ろまんじ variety that ended up deleted is that 
those weren't the type of misspellings that native Japanese speakers actually do. They weren't about documenting the language as 
it's actually used. Having にじゅうにち here, on the other hand, is not about accommodating learners making mistakes, it's about 
showing that Japanese people actually do pronounce it this way too (and some even arguing it's as correct as はつか). I do think it 
should be included here, but if we don't want to do that, I don't think we should have it as a separate "incorrect" entry either.

I'm very wary of having にじゅうにち even with an "ik" tag. I could accept it in a separate "mis-spelling" entry, but as with our former ろにん, ロマンジ, こにちわ, etc. entries we've gone away from that approach.
I suggest dropping it.

https://zatugaku-gimonn.com/entry2678.html
では二十日の正しい読み方は、「はつか」「にじゅうにち」どちらなのかを見ていきましょう。
結論から言ってしまうと、二十日の正しい読み方は「はつか」「にじゅうにち」の両方になります。
ただ一般的には二十日は「はつか」と読むことが多いです。
http://prime-com.jp/vip応対スキル/763/
電話応対のおもてなしルール②
...
聞き違えがないようにするためには
4月は「よんがつ」、7月は「なながつ」、
5日は「ごにち」、8日は「はちにち」、
20日は「にじゅうにち」というように
言葉を言い換えます。

https://twitter.com/koronekohs/status/676364433673818112
20日（にじゅうにち）って今までずっと言ってて、親に「それは間違ってる。一般常識では、はつかって言うよ」って言われて喧嘩→もうしんどいって言わ
れて曖昧に。
一般常識がどうのって言われたけど伝わればいいやんって何回も言ってんのに・・・最後に頭が硬いって言われました。ﾋﾟｷｯ

にじゅうにち is not in the dictionaries though wiki lists it and there's plenty of evidence and debate about it online. 
can't find anything in a reliable source though. probably safest (considering our users) to tag it as ik.

@@ -21,0 +22,4 @@
+</r_ele>
+<r_ele>
+<reb>にじゅうにち</reb>
+<re_inf>&ik;</re_inf>

@@ -3,0 +4,4 @@
+<k_ele>
+<keb>２０日</keb>
+<ke_pri>spec1</ke_pri>
+</k_ele>
@@ -9,2 +13 @@
-<keb>２０日</keb>
-<ke_pri>spec1</ke_pri>
+<keb>二〇日</keb>
@@ -22 +25 @@
-<gloss>twentieth day of the month</gloss>
+<gloss>20th day of the month</gloss>
@@ -26 +29 @@
-<gloss>twenty days</gloss>
+<gloss>20 days</gloss>